1. Field of the Invention
The present invention relates to an exposure control apparatus for use in a camera with an electronic flash apparatus. The present invention is also concerned with a camera of the type in which flashing of the electronic flash apparatus is controlled by means of the TTL automatic illumination adjusting method, and more particularly, with a camera of the type in which an illumination adjusting level is adjusted in accordance with the type of the electronic flash apparatus used in the camera.
2. Related Background Art:
In the conventional exposure control apparatus of the above-described type, an exposure value is calculated on the basis of the output of a photometric means for measuring the luminance of an object before an exposure operation is started, and an exposure operation is then conducted with an aperture and a shutter speed which are determined in accordance with the calculated exposure value.
Furthermore, in the flashing mode in which a shutter speed is normally fixed, an aperture corresponding to the already determined fixed shutter speed is first determined in accordance with the exposure value calculated on the basis of the photometric output. Next, the amount of the light from an object is measured Flashing is automatically stopped when the measured value reaches a predetermined level. In consequence, a correct exposure can be obtained for a background as well as for a person who is within the effective distance of the flash apparatus, which distance is determined by a guide number thereof.
Next, a TTL automatic illumination adjusting device for performing an automatic illumination adjustment control will be described with reference to FIG. 1 which shows a camera with the TTL automatic illumination adjusting unit. A camera body 1 incorporates a lens 2. An attachable electronic flash apparatus 3 is attached to the camera body 1. The camera body 1 shown is in an operating state. In this state, a main mirror 4 is raised, and a light coming from the electronic flash apparatus 3 and reflected by an object is passed through the taking lens 2. This light is reflected by a film surface 5, and is incident on a light-receiving element 7 through a light gathering lens 6. The light received by the light-receiving element 7 is converted to an electric signal, and this electric signal is integrated by time to obtain an integration value. An illumination adjusting circuit (not shown) outputs a flash stopping signal to stop flashing of the electronic flash apparatus 3 when this integration value reaches a level which is predetermined by the illumination adjusting circuit. A reference numeral 8 denotes a pentagonal prism, 9 denotes an aperture, and 10 denotes a shutter.
The flash apparatus for a camera is classified into two types: an attachable flash apparatus which can be attached to the camera body and a built-in type flash apparatus which is built in the camera body.
Generally, the attachable flash apparatus has a guide number of 20 or above, i.e., a long effective distance, and emits a large amount of light. The built-in type flash apparatus is generally designed to have a guide number of 15 or less, i.e., a short effective distance, and to emit a small amount of light because of the restricted space in the camera body where it is accommodated.
However, with a conventional exposure control unit, the same operation is performed to obtain an exposure value regardless of whether the type of flash apparatus is the attachable flash apparatus having a large guide number or the built-in type flash apparatus having a small guide number. In consequence, when an object located at a long distance is illuminated by the built-in flash apparatus having a small guide number, the amount of light emitted from the flash apparatus is small, and an underexposure is therefore obtained for the overall picture.
In particular, when a picture of a person who is beyond the effective distance of the flash apparatus is taken using the built-in flash apparatus under a dim available light, such as at dusk, the amount of light illuminating the person is small, and an underexposure is therefore obtained for the person. The background is illuminated by the light emitted from the flash apparatus as well as the available light, and a correct exposure is therefore obtained. This results in underexposure for the overall picture.
When a picture of a person who is in the shade is taken in the daytime using the built-in flash apparatus, and the distance between the person and the camera is long, underexposure of the person is likely to occur.
Furthermore, although the above-described conventional camera of the TTL automatic illumination adjusting type can employ a plurality of electronic flash apparatuses which generate different flashing waveforms or have different illumination stopping capabilities (the capabilities determined by the time required to actually stop the flashing after the flashing stopping signal has been sent out), the above-described illumination adjusting level is fixed. In consequence, the amount of light illuminating an object of given luminance may differ depending on the electronic flash apparatus used; a correct exposure may be obtained when a certain type of electronic flash apparatus is used, while underexposure or overexposure may be obtained when another type of electronic flash apparatus is used. In this specification, the flashing waveform or the flashing stopping capability is hereinafter referred to as the flashing characteristics.
The flashing characteristics will be described below in detail with reference to FIGS. 2 to 4.
FIG. 2 shows how the amount of light illuminating an object differs depending on the time required for a flash apparatus to actually stop flashing after a flashing stopping signal has been sent to the electronic flash apparatus. The ordinate axis represents an intensity of light I, and the abscissa axis represents a time t.
In the graph shown in FIG. 2, t0 is the time at which flashing is started, t1 is the time at which the flashing stopping signal is sent out, t2 and t3 are the times at which flashing of the flash apparatus is actually stopped. Since the amount of light illuminating the object is an integration value, if the period of time from time t1 to time t2 and the period of time from time 1 to time 3 differ from each other, the amount of light illuminating the object differs by the value indicated by the hatched area.
FIG. 3 is a graph, showing how the amount of light illuminating the object differs depending on the flashing waveforms.
Even though flashes with waveforms A and B stop at the same time t2' after the flashing stopping signal has been sent out at time t1', the amount of light illuminating the object differs by the value indicated by the hatched area.
In practice, the amount of light illuminating the object differs depending on the factors shown in FIGS. 2 and 3.
In a case where a picture is taken by the daylight synchronized flash, an object is illuminated by a smaller amount of light to enable a natural picture to be obtained. This makes the above-described problem more serious. For example, in the case shown in FIG. 4 which employs the same flashing waveform as that employed in the case shown in FIG. 2, the amount of light illuminating the object when the picture is taken by the daylight synchronized flash is reduced by sending out a flash stopping signal at time t1" which is earlier than time t1 in the case shown in FIG. 2. If it is assumed that t2"-t1"=t2-t1 and that t3"-t1"=t3-t1, the differences in the amounts of light irradiating the object in FIGS. 4 and 2 are respectively equal to the differences in the integration values between t3"-t2" and t3-t2. In consequence, the difference in the amount of light which occurs when the picture is taken by the daylight synchronized flash, as shown in FIG. 4, is greater because the flashing is stopped when the object is illuminated by a larger amount of light. That is, the hatched area is greater than that shown in FIG. 2.